Displacement of Mini-implants under Orthodontic Force Loading: A Systematic Review

INTRODUCTION

Anchorage planning is a priority when orthodontic treatment is being done. Skeletal anchorages have drawn the orthodontists’ attention for the same. Mini-implants (MIs) are easier to place than mini-plate¹ which require a surgical procedure for placement and removal.¹ Their small size allows them to be placed almost anywhere, with only minimal surgery for placement and the surgical procedure is minor enough for evasion of inflammation.^2,3 Mini-implants thus provide maximal anchorage control and requires minimal patient compliance.^4,5

The success rate of MIs is often called survival rate or stability. MIs are stabilized by mechanical interlocking of the cortical bone around the MI. Mini-implants offer the advantage of low cost and the placement of MI can be done in a single chairside clinical procedure. Mini-implants can be loaded immediately as there is no waiting period to allow for osseointegration.⁶

When an anchorage is adequately planned against orthodontic forces, teeth can be moved sufficiently. When anchorage planning is not done adequately then there will be reciprocal movement of the anchorage unit.⁷ There is evidence that suggests that when the orthodontic forces are applied onto MIs they drift and migrate under loading.⁸ Shorter MIs might be expected to subject the patient to less risk and discomfort during placement, but their stability remains to be established.² Primary stability is defined as implant stability immediately after insertion in the bone and is due to the mechanical contact between implant and bone, and also is dependent on factors like implant design, insertion angle, and bone density.^9–13

In orthodontic treatment, the MIs when used as an anchorage unit are under constant load over a long period. Creep is a time-dependent viscoelastic displacement of bone under a constant load. Thus, it is likely that the viscoelastic creep following the elastic static displacement is an active form of displacement that occurs in the bone surrounding a MI under the functional orthodontic constant loading in the clinic.¹⁴ Secondary displacement occurs over a treatment time and this can attribute to clinical scenarios where the MIs are in close root proximity and can even root resorption.¹⁵

Various pieces of evidence in the literature have reported a progressive migration of the implants upon orthodontic loading.¹⁶ The main objective is to assess the clinical trials and prospective studies that evaluate the displacement, migration, or mobility of orthodontic MIs on the application of orthodontic force. The secondary outcomes that are assessed in this study are the mobility and failure of MIs.

This systematic review aims at the displacement of MIs by evaluating how strong is the mechanical interlocking of the MIs with the cortical bone and dislodgement or the drift characteristics of the MIs on the application of orthodontic force. The null hypothesis of this systematic review is that there is no displacement of MI when loaded for orthodontic treatment and the alternate hypothesis is there is a displacement of MI when being loaded for orthodontic treatment. The objective of the study was to assess the displacement, mobility, and root approximation of the MIs which were used as skeletal anchorage and were loaded during orthodontic treatment.

MATERIALS AND METHODS

This trial has been registered to PROSPERO (International Prospective Register of Systematic Reviews) and the registration number is CRD42020150084 in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist of systematic reviews and meta-analyses.

Two independent authors (SS and AKS) screened the initial titles and abstracts to find all the eligible studies from 1950 until June 26, 2020, from PubMed, Cochrane library, Google Scholar Beta, and LILACS using the terms Malocclusion, orthodontic treatment, orthodontic tooth movement, orthodontic therapy, orthodontic appliance, orthodontic patients, temporary anchorage devices, Screw, mini-screw, mini-implant, skeletal anchorage, orthodontic anchorage, anchor loss, loading behavior, reactive force, stability, primary displacement, migration, dislodgement, loss of anchorage drift, primary stability, loosening, drift characteristics, movement, deflections, biomechanical effect, randomized controlled trial, NOT functional appliance, corticotomy, and microosteoperforation. The full texts were retrieved according to their inclusion and exclusion criteria. All differences of opinions were discussed and resolved. If necessary, the third author (NR) was consulted. The results were screened with title and abstract screening to select which studies will be included in this review. The references used in these studies were hand-searched to see if there were any clinical trials included.

The data were collected that were included based on the author’s name, publication year, study type, subjects, interventions, age group, treatment time, method of measurements, and outcomes assessed. The data of studies that assessed MI displacement, mobility, and failure rates in MIs placed in subjects at various locations for skeletal anchorage were considered.^5,8,15–24 Two independent authors assessed the studies included for the studies and were discussed with the third author.

The eligibility criteria were defined based on the PICO research strategy for clinical practice based on scientific evidence. The inclusion criteria for this review included participants undergoing fixed orthodontic treatment and that use MI, the types of study designs included were randomized controlled trials, prospective and retrospective clinical trial; and the outcome measures taken into consideration were displacement and mobility of MIs under orthodontic force loading. The exclusion criteria for this systematic review were studies that were done in vitro or using FEM analysis or in animals, studies that used other modes of skeletal anchorage other than MIs, and studies that were conducted on patients below 14 years and older than 50 years.

Based on the exclusion criteria, we have excluded the studies that assessed the displacement of MIs but were not performed as clinical trials and did not assess these results in patients and were tested in artificial environment or simulation studies.^{1,2,7,9,14,25–33}

Two independent authors assessed the risk of bias for all the studies included. A fourth author was asked for advice and the final decision was made.

The PRISMA flow diagram of the selection of studies is represented (Flowchart 1). The search strategy helped us retrieve the following number of records from databases: PubMed (n = 81), Google scholar (n = 48), and Cochrane (n = 3) totalling 132 studies. After deletion of duplicates, there were 79 records for the title and abstract reading of which a total of 45 were eliminated as they were not relevant to the topic or did not meet the inclusion criteria. After reading the full article, 12 articles were chosen for qualitative analysis (Table 1). Articles could not be considered for a quantitative meta-analysis given the heterogeneity of the included studies.

Randomized trials were assessed using the Cochrane Risk of Bias (RoB 2.0) tool, Higgins JPT 2016 which involves judgment on seven headings as formulated by the Cochrane Group.³⁴ The risk of bias for each of the domains and overall risk of bias was made as per the recommendations of the RoB 2.0 tool. Trials were classified overall as having a low risk of bias, some concerns of bias, or a high risk of bias as described in the RoB 2.0 tool. Non-randomized trials were assessed on Newcastle–Ottawa quality assessment scale.³⁵ The risk of bias for each of the headings and overall risk of bias was made as per the recommendations of the ROBINS-I tool.³⁶ Trials were classified overall as having no information, low risk, moderate risk, serious risk, or critical risk of bias (Fig. 1).

The possible influence of small study publication biases on review findings was considered and formed a part of the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) level of evidence (GRADEpro Guideline Development Tool, available online at gradepro.org).³⁷ The influence of small study biases was addressed by the risk of bias criterion “study size”. Assessment of the quality of the body of evidence-based on Oxford’s CEBM table.³⁸

RESULTS

DISCUSSION

Absolute anchorage is when there is no movement of the placed MI due to the reactionary force that is applied to move the teeth.³⁹ Skeletal anchorage aims at achieving an absolute anchorage. In this review, we aim at assessing if the MIs or screws used as skeletal anchorage in orthodontics migrated or displaced as a reciprocal reaction to the force applied. According to Newton’s law which states that for every action in nature there is an equal and opposite reaction. It is nature’s action to have a reactionary force on the MI when another force is applied to the implant in the opposite direction of its mechanical engagement to the bone.

Many factors are the cause for MI displacement. Mini-implants require some kind of surgical intervention and no implant can be directly placed and force applied through elastics.⁴⁰

The implant factors that affect the rate of stability of an implant can be numerous. Implants can be self-drilling or self-tapping MIs and in the maxilla, there has been reported more successful use of self-tapping, as there are higher failure rates and more chance of mobility in self-drilling.⁴¹ In orthodontic MIs, there have been attempts made to modify the implant surface like sandblasting, etching to improve their stability throughout the treatment period.¹⁸ This is also based on the fact that this anchorage influences the orthodontic treatment and tooth movement.⁴² Motoyoshi et al. stated that abutment was effective in raising initial implant stability.⁴³ Hedayati et al. extensively studied the MIs and stated that titanium screws are better for anchorage as in orthodontic treatment the MIs are loaded for 6–7 months and continuously loaded to 150–200 g.⁵

The next set of factors to be taken into consideration about the stability of implant is its placement-related factors. Liu et al. suggested that a mesial site for MI placement will be a better choice for its long-term stability.²¹ Łyczek et al. conducted a study that tested antibiotic prophylaxis for MI success but they did not report any statistically significant success.²² El-Beialy et al. reported that the vertical placement angle and the length of the MI did not determine its success.¹⁹ If a drill needs to be used to place MIs, it is better to place with smaller diameter drills to contribute to better clinical stability.²⁴ Migliorati et al. noted that torque values play an important role in the stability of the MIs; early torque decrease after implant placement suggests relaxation phenomenon and the implant is stable for a longer period.²⁶ Higher values of torque may result in higher failure rates because of bone compression and microdamage and can also cause fracture of the MIs. Chatzigianni et al. stated that at higher force levels, longer and wider MIs are less displaced.⁹ Brettin et al. suggested that bicortical MIs for better resistance when compared with monocortical skeletal anchorage.³¹ Pickard et al. stated that even though the MI is only as small as 6 mm, there are chances that the lingual cortex is reached during placement, for maximum stability and resistance to failure the MI should be loaded along its long axis.³⁰

Park et al. explained various factors such as the host and the environment that determine the stability of the MIs.¹³ They reported more mobility and failure in the right side and the mandible when compared to the left side and maxilla. Lee et al. reported the effect of hygiene in the right and left side due to right-handedness and the better the hygiene there is less chance of inflammation which allows for a better chance of success of the orthodontic MIs.¹² Park et al. commented on the direction of loading as an important factor that influences if the implant gets displaced on loading.¹³ Singh et al. found out that there is a winding movement of MI upon orthodontic loading.³³

On assessing the displacement or the absoluteness of the anchorage, Moon et al. concluded that most displacements of orthodontic MIs happened in the initial 2 months and failure occurred within 4 months.²⁷ Alves et al. studied the migration of MIs by measuring the movement of the head and the tail of the MIs under orthodontic force loading and that the displacement was of less clinical relevance regarding its dimensions and the mechanics of movement.¹⁷ Kinzinger et al. stated that titanium MIs did not provide a stable anchorage for molar distalization.²⁰ Park et al. stated that even slightly mobile MIs could withstand force application below 200 cN.¹³ Wang et al. stated that there was movement in both the predrilling and self-drilling MIs on orthodontic force loading.¹⁶ The pattern of the displacement or the movement of MI is extrusion, controlled or uncontrolled tipping or bodily movement, and also the displacement of the MI on the right and left side of the same patient need not be the same. The longer the loading period on the MI, the more horizontal displacement occurred, the MI should be placed with a 2 mm of clearance from the tooth roots and the surrounding vital structures.

The limitation of this study is that this review provides an insufficient explanation. From the evidence we assessed, on force application on MI causes dislodgement and drifting of mini-implants and hence it would be safer to evaluate the inter-radicular space before MI is being planned for skeletal anchorage, but no quantitative data can be given on the amount of displacement. Further reviews are needed to evaluate the individual factors that attribute to the displacement of MIs and the success of MIs. More well-planned clinical trials involving MIs success and its factors need to be assessed.

From this systematic review, we can conclude that the MIs provide adequate anchorage when an absolute anchorage is planned in orthodontics. There is primary displacement or mobility that occurs and from these included studies we can confirm that a significant level of secondary displacement and migration of the MI occurs on functional orthodontic loading. Considering that there is a reciprocal movement of the MI, the direction, and position of insertion of the MI should be planned.

CONCLUSION

From evaluating the articles included based on our inclusion criteria in this review, we conclude that there is a displacement of the MI under orthodontic loading and application of force. The studies that we assessed have different methods of evaluation and these warrants for randomized controlled trials with an adequate sample size by estimation of the power of the study, proper randomization technique, and a standard technique for assessing the displacement of the MI will improve the strength of evidence. We can conclude that MIs provide adequate anchorage during orthodontic treatment because the primary displacement of the MIs did not appear to be clinically relevant to failure and mobility.

REFERENCES

1. Ohmae M, Saito S, Morohashi T, et al. A clinical and histological evaluation of titanium mini-implants as anchors for orthodontic intrusion in the beagle dog. Am J Orthod Dentofac Orthoped 2001;119(5):489–497. DOI: 10.1067/mod.2001.114300.

2. Mortensen MG, Buschang PH, Oliver DR, et al. Stability of immediately loaded 3- and 6-mm miniscrew implants in beagle dogs—a pilot study. Am J Orthod Dentofac Orthoped 2009;136(2):251–259. DOI: 10.1016/j.ajodo.2008.03.016.

3. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763–767.

4. Favero L, Brollo P, Bressan E. Orthodontic anchorage with specific fixtures: related study analysis. Am J Orthod Dentofac Orthoped 2002;122(1):84–94. DOI: 10.1067/mod.2002.124870.

5. Hedayati Z, Hashemi SM, Zamiri B, et al. Anchorage value of surgical titanium screws in orthodontic tooth movement. Int J Oral Maxillofac Surg 2007;36(7):588–592. DOI: 10.1016/j.ijom.2006.10.020.

6. Yanosky MR, Holmes JD. Mini-implant temporary anchorage devices: orthodontic applications. Compend Contin Educ Dent 2008;29(1):12–20.

7. Motoyoshi M, Yano S, Tsuruoka T, et al. Biomechanical effect of abutment on stability of orthodontic mini‐implant: a finite element analysis. Clin Oral Implants Res 2005;16(4):480–485. DOI: 10.1111/j.1600-0501.2005.01130.x.

8. Liou EJ, Pai BC, Lin JC. Do miniscrews remain stationary under orthodontic forces? Am J Orthod Dentofac Orthoped 2004;126(1):42–47. DOI: 10.1016/j.ajodo.2003.06.018.

9. Chatzigianni A, Keilig L, Reimann S, et al. Effect of mini-implant length and diameter on primary stability under loading with two force levels. Eur J Orthod 2011;33(4):381–387. DOI: 10.1093/ejo/cjq088.

10. Min KI, Kim SC, Kang KH, et al. Root proximity and cortical bone thickness effects on the success rate of orthodontic micro-implants using cone beam computed tomography. Angle Orthod 2012;82(6):1014–1021. DOI: 10.2319/091311-593.1.

11. Jung YR, Kim SC, Kang KH, et al. Placement angle effects on the success rate of orthodontic microimplants and other factors with cone-beam computed tomography. Am J Orthod Dentofac Orthoped 2013;143(2):173–181. DOI: 10.1016/j.ajodo.2012.09.011.

12. Lee MY, Park JH, Kim SC, et al. Bone density effects on the success rate of orthodontic microimplants evaluated with cone-beam computed tomography. Am J Orthod Dentofac Orthoped 2016;149(2):217–224. DOI: 10.1016/j.ajodo.2015.07.037.

13. Park JH, Chae JM, Bay RC, et al. Evaluation of factors influencing the success rate of orthodontic microimplants using panoramic radiographs. Korean J Orthod 2018;48(1):30. DOI: 10.4041/kjod.2018.48.1.30.

14. Pittman JW, Navalgund A, Byun SH, et al. Primary migration of a mini-implant under a functional orthodontic loading. Clin Oral Investigat 2014;18(3):721–728. DOI: 10.1007/s00784-013-1045-9.

15. Lee YK, Kim JW, Baek SH, et al. Root and bone response to the proximity of a mini-implant under orthodontic loading. Angle Orthod 2010;80(3):452–458. DOI: 10.2319/070209-369.1.

16. Wang YC, Liou EJ. Comparison of the loading behavior of self-drilling and predrilled miniscrews throughout orthodontic loading. Am J Orthod Dentofac Orthoped 2008;133(1):38–43. DOI: 10.1016/j.ajodo.2006.01.042.

17. Alves M,Jr Baratieri C, Nojima LI. Assessment of mini‐implant displacement using cone beam computed tomography. Clin Oral Implants Res 2011;22(10):1151–1156. DOI: 10.1111/j.1600-0501.2010.02092.x.

18. Calderón JH, Valencia RM, Casasa AA, et al. Biomechanical anchorage evaluation of mini-implants treated with sandblasting and acid etching in orthodontics. Implant Dentis 2011;20(4):273–279. DOI: 10.1097/ID.0b013e3182167308.

19. El-Beialy AR, Abou-El-Ezz AM, Attia KH, et al. Loss of anchorage of miniscrews: a 3-dimensional assessment. Am J Orthod Dentofac Orthoped 2009;136(5):700–707. DOI: 10.1016/j.ajodo.2007.10.059.

20. Kinzinger G, Gülden N, Yildizhan F, et al. Anchorage efficacy of palatally-inserted miniscrews in molar distalization with a periodontally/miniscrew-anchored distal jet. J Orofac Orthopedics/Fortschritte der Kieferorthopädie 2008;69(2):110–120. DOI: 10.1007/s00056-008-0736-3.

21. Liu H, Lv T, Wang NN, et al. Drift characteristics of miniscrews and molars for anchorage under orthodontic force: 3-dimensional computed tomography registration evaluation. Am J Orthod Dentofac Orthoped 2011;139(1):e83–e89. DOI: 10.1016/j.ajodo.2010.07.018.

22. Łyczek J, Kawala B, Antoszewska-Smith J. Influence of antibiotic prophylaxis on the stability of orthodontic microimplants: a pilot randomized controlled trial. Am J Orthod Dentofac Orthoped 2018;153(5):621–631. DOI: 10.1016/j.ajodo.2017.11.025.

23. Park HS, Jeong SH, Kwon OW. Factors affecting the clinical success of screw implants used as orthodontic anchorage. Am J Orthod Dentofac Orthoped 2006;130(1):18–25. DOI: 10.1016/j.ajodo.2004.11.032.

24. Son S, Motoyoshi M, Uchida Y, et al. Comparative study of the primary stability of self-drilling and self-tapping orthodontic miniscrews. Am J Orthod Dentofac Orthoped 2014;145(4):480–485. DOI: 10.1016/j.ajodo.2013.12.020.

25. Ganzer N, Feldmann I, Bondemark L. Anchorage reinforcement with miniscrews and molar blocks in adolescents: a randomized controlled trial. Am J Orthod Dentofac Orthoped 2018;154(6):758–767. DOI: 10.1016/j.ajodo.2018.07.011.

26. Migliorati M, Drago S, Gallo F, et al. Immediate versus delayed loading: comparison of primary stability loss after miniscrew placement in orthodontic patients—a single-centre blinded randomized clinical trial. Eur J Orthod 2016;38(6):652–659. DOI: 10.1093/ejo/cjv095.

27. Moon CH, Lee DG, Lee HS, et al. Factors associated with the success rate of orthodontic miniscrews placed in the upper and lower posterior buccal region. Angle Orthod 2008;78(1):101–106. DOI: 10.2319/121706-515.1.

28. Scannell J, Measurement of applied force to dislodge orthodontic temporary anchorage devices (Doctoral dissertation, University of Birmingham). 2012.

29. Wilmes B, Drescher D. Impact of insertion depth and predrilling diameter on primary stability of orthodontic mini-implants. Angle Orthod 2009;79(4):609–614. DOI: 10.2319/071708-373.1.

30. Pickard MB, Dechow P, Rossouw PE, et al. Effects of miniscrew orientation on implant stability and resistance to failure. Am J Orthod Dentofac Orthoped 2010;137(1):91–99. DOI: 10.1016/j.ajodo.2007.12.034.

31. Brettin BT, Grosland NM, Qian F, et al. Bicortical vs monocortical orthodontic skeletal anchorage. Am J Orthod Dentofac Orthoped 2008;134(5):625–635. DOI: 10.1016/j.ajodo.2007.01.031.

32. Jang HJ, Kwon SY, Kim SH, et al. Effects of washer on the stress distribution of mini-implant: a finite element analysis. Angle Orthod 2012;82(1):137–144. DOI: 10.2319/021411-107.1.

33. Singh S, Mogra S, Shetty VS, et al. Three-dimensional finite element analysis of strength, stability, and stress distribution in orthodontic anchorage: a conical, self-drilling miniscrew implant system. Am J Orthod Dentofac Orthoped 2012;141(3):327–336. DOI: 10.1016/j.ajodo.2011.07.022.

34. Higgins JPT, Altman DG, Gotzsche PC, et al. The Cochrane Collaboration’s tool for assessing risk of bias in randomised trials. BMJ 2011;343(oct18 2):d5928. DOI: 10.1136/bmj.d5928.

35. Stang A. Critical evaluation of the Newcastle-Ottawa scale for the assessment of the quality of nonrandomized studies in meta-analyses. Eur J Epidemiol 2010;25(9):603–605. DOI: 10.1007/s10654-010-9491-z.

36. Sterne JA, Hernán MA, Reeves BC, et al. ROBINS-I: a tool for assessing risk of bias in non-randomised studies of interventions. BMJ 2016. 355. DOI: 10.1136/bmj.i4919.

37. GRADEpro GD. GRADEpro guideline development tool [software]. McMaster University 2015. 435.

38. Heneghan C. EBM resources on the new CEBM website. BMJ Evidence-Based Med 2009;14(3):67. DOI: 10.1136/ebm.14.3.67.

39. Papadopoulos MA, Tarawneh F. The use of miniscrew implants for temporary skeletal anchorage in orthodontics: a comprehensive review. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;103(5):e6–e15. DOI: 10.1016/j.tripleo.2006.11.022.

40. Vikram NR, Prabhakar R, Kumar SA, et al. Ball headed mini implant. J Clin Diagnos Res 2017;11(1):ZL02. DOI: 10.7860/JCDR/2017/24358.9240.

41. Çehreli S, Arman-Özçırpıcı A. Primary stability and histomorphometric bone-implant contact of self-drilling and self-tapping orthodontic microimplants. Am J Orthod Dentofac Orthoped 2012;141(2):187–195. DOI: 10.1016/j.ajodo.2011.07.020.

42. Costa A, Raffainl M, Melsen B. Miniscrews as orthodontic anchorage: a preliminary report. Int J Adult Orthod Orthognat Surg 1998;13(3):201–209.

43. Motoyoshi M, Yoshida T, Ono A, et al. Effect of cortical bone thickness and implant placement torque on stability of orthodontic mini-implants. Int J Oral Maxillofac Implants 2007;22(5):779–784.

44. Türköz Ç, Ataç MS, Tuncer C, et al. The effect of drill-free and drilling methods on the stability of mini-implants under early orthodontic loading in adolescent patients. Eur J Orthod 2011;33(5):533–536. DOI: 10.1093/ejo/cjq115.